In recent years, there has been a need for an optical recording medium capable of high-density and high-speed recording, and as one of concrete measures for responding to such need, an optical recording medium capable of recording and reproducing by means of blue wavelength laser beam is considered noticeable.
Among such optical recording media utilizing blue wavelength laser beam, optical recording media of the read only memory (ROM) type and optical recording media of the rewritable (RW) type have already been proposed.
However, there has been no proposal yet of a write-once type of optical recording medium capable of high speed recording utilizing laser beam with a wavelength shorter than blue wavelengths.
In one of recording structures for write-once type optical recording media that have been conventionally proposed, an organic dye is applied on a substrate. This structure utilizes the fact that the dye in a part irradiated with laser beam absorbs light and generates heat, by which the configuration or refractive index of the substrate is changed, and the part thus changed is read as the change in reflectance.
However, the recording sensibility of recording media using an organic dye is not high enough to perform high-speed recording. Also, if the laser beam wavelength is made shorter for increasing the recording density, it becomes difficult to synthesize dye that can be used with laser beam particularly with a wavelength shorter than blue wavelengths.
Although proposals have been made to form a recording layer from an inorganic material (See Japanese Patent Publication No. 1992-838, for example), all of these conventional structures pose many problems such that they are not suitable for high-speed recording, their reliability in storage stability in the recorded state is not sufficiently high, and their reproduction durability is poor.
Under such circumstances, presently, there exists no write-once type optical recording medium that has high storage stability, is low-cost, and is capable of high-speed and high-density recording/storing.
Generally speaking, if the read power (the power of laser beam irradiated for reproducing recorded information) Pr is low, the ratio of noise to reproduced signal becomes relatively high to decrease the so-called CNR (carrier-to-noise ratio), and the reliability for reproduction is lowered by that much. On the other hand, if the write power (the power of laser beam irradiated for recording information) Pw is high, laser radiation equipment with higher output by that much is required, and the costs of parts and devices are increased. Further, the lifetime of laser tends to become shorter.
Therefore, in terms of the viewpoint of hardware such as disk drive, it is qualitatively preferable that the read power Pr is high and the write power Pw is low. In other words, the ratio Pw/Pr is desirably as close as possible to 1.
However, in conventional optical recording media, the ratio of write power Pw to read power Pr, Pw/Pr had to be set to “10 or more” for the reasons of securing stability in recording and reproduction, ensuring long term reliability and so on.